The present invention relates generally to the field of chemical vapor deposition (CVD) in semiconductor device manufacturing, and more specifically to a method for cleaning and preconditioning a dome in a CVD system.
Chemical vapor deposition (CVD) processes are used widely in the manufacture of semiconductor devices. Generally, CVD involves exposing a semiconductor wafer to a reactive gas under carefully controlled conditions including elevated temperatures, sub-ambient pressures and uniform reactant gas flow rate, resulting in the deposition of a thin, uniform layer or film on the surface of the substrate. The undesired gaseous byproducts of the reaction are then pumped out of the deposition chamber. The CVD reaction may be driven thermally or by a reactant plasma, or by a combination of heat and plasma. CVD systems in which the reaction is driven by a reactant plasma are known as plasma-assisted or plasma-enhanced.
A typical plasma-enhanced CVD system is a single-wafer system utilizing a high-throughput CVD chamber. The chamber typically comprises an aluminum oxide (Al2O3) dome, an induction coil that is provided in an expanding spiral pattern adjacent to the dome and outside the chamber, at least one gas injection nozzle, and a vacuum pump for evacuating the gaseous byproducts of the CVD process from the chamber. RF power is applied to the induction coil to generate a reactive plasma within the chamber. A cooling fluid, such as cooling water, is also typically flowed through the induction coil, either from the bottom of the dome to the top of the dome, or from the top of the dome to the bottom of the dome.
A typical CVD process begins with heating of the CVD chamber. A semiconductor substrate is placed in the chamber on a receptor, also known as a susceptor, which is typically made of ceramic or anodized aluminum. Next, reactant gases are introduced into the chamber, while regulating the chamber pressure. The gases react in the chamber to form a deposition layer on the surface of the wafer.
In a typical deposition process, reactant gases enter the reaction chamber and produce films of various materials on the surface of a substrate for various purposes, such as for dielectric layers, insulation layers, etc. The various materials deposited include epitaxial silicon, polysilicon, silicon nitride, silicon oxide, and refractory metals such as titanium, tungsten and their suicides. Much of the material produced from the reactant gases is deposited on the wafer surface. However, some material also is inevitably deposited on other surfaces inside the chamber. These deposits must be removed periodically to prevent them from building up to the point where particulate contamination is generated, which can cause opens or shorts in the microelectronic device.
The material which is deposited on the interior chamber walls is usually deposited with a nonuniform thickness. This nonuniformity in thickness results in part from a temperature gradient in the dome produced by cooling water flowing through the induction coil. For example, if cooling water enters the induction coil at the bottom of the dome and exits at the top of the dome, then the dome will be slightly cooler at the bottom than at the top, because the cooling water is gradually heated as it flows through the coil. This temperature gradient within the dome will result in a deposition thickness which is greater at the bottom than at the top.
In a typical CVD system, after one or more deposition processes wherein a film is deposited onto a semiconductor substrate and the substrate is removed from the chamber, a cleaning gas or mixture of cleaning gases is purged through the reaction chamber in order to clean unwanted deposits from the chamber interior surfaces, including the chamber walls and the susceptor. For cleaning silicon dioxide (SiO2) films from the chamber interior, a typical cleaning gas system comprises fluorinecontaining gases, such as a mixture of nitrogen trifluoride (NF3) and hexafluoroethane (C2F6). A plasma gas is typically ignited in the chamber to enhance the efficiency of the cleaning gas mixture. The plasma creates fluorine radicals which react with the SiO2 film under the influence of ion bombardment to form SiF4 and other volatile compounds, which are then pumped out of the reaction chamber by the vacuum pump.
However, some of the fluorine-containing species in the cleaning gas also react with the Al2O3 in the dome to form AlF3, especially in areas of the dome where SiO2 coverage is thinner. For example, if the direction of cooling water flow is from the bottom of the dome to the top, then SiO2 coverage will be thinner at the top. Therefore, with uniform cleaning rates throughout the chamber, the SiO2 at the top of the dome will be removed before it is completely removed at the bottom of the dome. To achieve complete removal of deposited SiO2 at the bottom of the dome, the top of the dome will be xe2x80x9cover-cleaned,xe2x80x9d resulting in conversion of Al2O3 at the top of the dome to AlF3.
This problem of over-cleaning is exacerbated by the temperature gradient in the dome discussed previously. If the direction of cooling water flow is from the bottom of the dome to the top, then the dome is slightly cooler at the bottom, resulting in poorer cleaning efficiency at the bottom. To achieve complete removal of deposited SiO2 at the bottom of the dome, over-cleaning to an even greater extent will result at the top of the dome.
Therefore, after each cleaning step, it is necessary to precondition or passivate the dome, in order to convert the AlF3 species on the dome back to Al2O3. A typical preconditioning process comprises introducing H2, and then a mixture of H2 and O2 into the chamber. It is believed that the initial amount of H2 reacts with AlF3 species on the dome to partially passivate the dome, producing some Al2O3 and intermediate AlyOx and AlyOxFz species. (In the variable stoichiometric formulas presented throughout this document, such as AlyOx and AlyOxFz, the variables x, y and z represent, independently, integer or fractional numbers, and may be the same or different.) The subsequent mixture of activated H2 and O2 reacts with these intermediate species to form Al2O3. Byproducts of this reaction include hydrofluoric acid (HF) and water. The production of water as a byproduct severely limits the passivation reaction, because water is a contaminant which is very difficult to remove once absorbed on the dome surface. In addition, HF combines with water to form aqueous HF, which is also a very difficult species to remove once absorbed. Therefore, the presence of water and aqueous HF prevent overpassivation and often result in incomplete passivation of the dome, leaving some of the AlF3 species on the dome.
The presence of some AlF3 species on the dome during subsequent processing in the CVD chamber can cause significant particulate contamination. During deposition, the AlF3 species may react with process gases to form gaseous byproducts, which will release particulate contamination. Specifically, it is believed that AlF3 reacts with process gases such as N2, SiH4 and H2 to form gaseous byproducts such as HF, SiF4 and NF3. The conversion of AlF3 to these gaseous byproducts causes the release of films which have been deposited on top of the AlF3 species. The release of these films creates particulate contamination, which can severely impact microelectronic device yield and reliability.
The problem of incomplete passivation is of particular concern when depositing silicon nitride (SiN) films. Because SiN forms a different bond with Al2O3 and AlF3 than does SiO2, particle formation as a result of weaker adhesion of subsequently deposited films during deposition is more likely. For example, particulate contamination may be generated as a result of stress-induced de-adhesion of pre-coat during the deposition of product.
The present invention eliminates the aforementioned problems by providing a method for cleaning and preconditioning the dome of a CVD chamber.
In a first aspect of the present invention, a method is provided for preconditioning a dome of a chemical vapor deposition chamber, comprising the steps of: introducing hydrogen gas into said chamber; generating a reactive plasma of said hydrogen gas in said chamber; introducing a mixture of hydrogen gas and nitrogen gas into said chamber; and generating a reactive plasma of said mixture of hydrogen gas and nitrogen gas in said chamber.
In another aspect of the present invention, a method is provided for cleaning a dome of a chemical vapor deposition chamber, wherein said dome is cooled during deposition by flowing a cooling fluid in a first direction through an induction coil having multiple windings provided in an expanding spiral pattern in said dome. The method comprises the steps of: flowing said cooling fluid in a second direction through said induction coil, such that said second direction is opposite said first direction; introducing at least one cleaning gas into said chamber; and generating a reactive plasma of said cleaning gas.
In yet another aspect of the present invention, a method is provided for cleaning and preconditioning a dome of a chemical vapor deposition chamber, wherein said dome is cooled during deposition by flowing a cooling fluid in a first direction through an induction coil having multiple windings provided in an expanding spiral pattern in said dome. The method comprises the steps of: flowing said cooling fluid in a second direction through said induction coil, such that said second direction is opposite said first direction; introducing at least one cleaning gas into said chamber; generating a reactive plasma of said cleaning gas; evacuating said cleaning gas from said chamber; flowing said cooling fluid in said first direction through said induction coil; introducing hydrogen gas into said chamber; generating a reactive plasma of said hydrogen gas in said chamber; introducing a mixture of hydrogen gas and nitrogen gas into said chamber; and generating a reactive plasma of said mixture of hydrogen gas and nitrogen gas in said chamber.
A preferred apparatus for cooling a dome of a chemical vapor deposition chamber is also provided, wherein said dome has a top and a bottom, and said dome comprises an induction coil which is comprised of copper tubing and has multiple windings provided in an expanding spiral pattern in said dome. The apparatus comprises a first connection point at a first end of said copper tubing, for connecting a cooling fluid supply to said copper tubing; a second connection at a second end of said copper tubing, for connecting a cooling fluid return to said copper tubing; first and second valves provided between and in fluid communication with said first connection point and said dome, wherein said first valve is in fluid communication with the top of said dome and said second valve is in fluid communication with the bottom of said dome; third and fourth valves provided between and in fluid communication with said second connection point and said dome, wherein said third valve is in fluid communication with the top of said dome, and said fourth valve is in fluid communication with the bottom of said dome.